1. Field of the Invention
The present invention relates to a fluorescent substance usable in a display such as a field emission display or in a light-emitting device comprising a blue or ultraviolet LED as a light source.
2. Background Art
LED lamps, which utilize light-emitting diodes, are used in many displaying elements of instruments such as mobile devices, PC peripheral equipments, OA equipments, various kinds of switches, light sources for backlighting, and indicating boards. The LED lamps are strongly required not only to have high efficiency, but also to be excellent in color rendition when used for general lighting or to deliver a wide color gamut when used for backlighting. In order to increase the efficiency, it is necessary to adopt a highly efficient fluorescent substance. Further, for improving the color rendition and for broadening the color gamut, it is effective to adopt a fluorescent substance that emits green luminescence under the excitation with blue light.
Incidentally, high load LEDs generally become so hot while working that fluorescent substances used therein are heated up to a temperature of approx. 100 to 200° C. When the fluorescent substances are thus heated, their emission intensity is generally lowered. Accordingly, it is desired that the emission intensity be less lowered even if the fluorescent substances are heated.
In view of the above, an Eu-activated β-SiAlON phosphor can be regarded as an example of the fluorescent substances which emit green luminescence under the excitation with blue light and which are suitably used for the aforementioned LED lamps. This phosphor emits luminescence efficiently when excited at 450 nm, and its absorption ratio, inner quantum efficiency and emission efficiency under the excitation at 450 nm are approx. 65%, 53% and 35%, respectively (JP-A 2005-255895(KOKAI)). These are also reported in N. Hirosaki et al., Extended Abstracts (The 53rd Spring meeting); The Japan Society of Applied physics and Related Societies, 25p-ZR-11 (2006).
Meanwhile, a lot of energetic work has recently been devoted to development of flat panel displays, and the development has put emphasis on the study of PDPs (plasma displays) and LCDs (liquid crystal displays). However, field emission displays are expected to give much clearer images and hence to be more advantageous than the PDPs and LCDs.
The field emission display comprises a screen on which red, green and blue fluorescent substances are arranged, and a cathode spaced apart from and facing to the screen. The space between the screen and the cathode is smaller than that in a CRT. The cathode includes plural emitter elements as electron sources, which emit electrons in accordance with potential difference between the emitter elements and gate electrodes placed nearby. The electrons thus emitted are accelerated by an anode voltage (accelerating voltage) applied on the fluorescent substance side, and then made to impinge against the fluorescent substances, so that the fluorescent substances give off luminescence to display a clear image.
As for the fluorescent substances used in the field emission display constituted as described above, it is required to have sufficiently high emission efficiency and to exhibit the high emission efficiency fully even when saturation is achieved by excitation with a high current density. From the viewpoint of only this requirement, sulfide phosphors (e.g., ZnS:Cu, ZnS:Ag), which are conventionally used as fluorescent substances for CRTs, may be considered as prospective candidates for the fluorescent substances usable in the field emission display. However, it is reported that the sulfide phosphors such as ZnS decompose under the condition where a low-energy cathode ray display screen is excited. This is a serious problem because the decomposed products thus generated badly degrade a heat filament, from which electron beams are emitted. Further, conventionally used ZnS-based blue fluorescent substances are more liable to deteriorate in luminance as compared with red and green fluorescent substances. This raises another problem that colors in displayed color images gradually fade or change with time.